In conventional FMCW radar sensors (frequency modulated continuous wave), the frequency is modulated in the form of a ramp within a fixed frequency band, for example alternately using a rising frequency ramp and a falling frequency ramp.
The received radar echo is mixed with a component of the transmission signal, so that an intermediate frequency signal is obtained, whose frequency corresponds to the frequency difference between transmission signal and received signal. A located object is shown in the spectrum of the intermediate frequency signal in the form of a peak, whose apex frequency is a function of the signal runtime and thus the distance of the object and, because of the Doppler effect, also the relative velocity of the object. By comparing the apex frequencies of two peaks, which originate from the same object and of which one was obtained on the rising frequency ramp and the other on the falling frequency ramp, the distance and the relative velocity of the object may be determined.
A monostatic antenna concept is common, in which the antenna elements are used both for transmitting and also for receiving.
Multiple antenna elements are typically positioned in the horizontal adjacent to one another in front of a shared microwave lens. The radar radiation of each antenna element is bundled by the lens and emitted in an established main emission direction, which is a function of the location of the relevant antenna element. In this way, a multibeam radar is obtained, which covers a larger angle range in the horizontal, i.e., in the azimuth, and allows the azimuth angle of the object to be determined by comparing the amplitudes and phases of the radar echoes received by various antenna elements.
Fundamentally, instead of a microwave lens or in combination therewith, group antennas may also be used for beam shaping and for establishing the emission direction, the group antennas having multiple individual antennas, which are actuated using different phases, so that the desired emission characteristic results from interference effects.
For example, German Patent Application No. DE 196 38 149 describes a group antenna, using which the emission direction may be influenced not only in the azimuth, but also in the elevation (in the vertical). The use of antenna elements having multiple subelements which are positioned in the vertical and serially fed is also described, whereby an antenna characteristic which varies as a function of the frequency of the transmission signal is obtained in the elevation.